Solid electrolyte, electrode mixture containing same, solid electrolyte layer, solid state battery, and method for evaluating solid electrolyte

Abstract

The present invention addresses the problem of providing a solid electrolyte that allows a solid state battery to be used safely even when the usage environment changes from a normal usage environment to a harsh usage environment. In an X-ray diffraction pattern of the solid electrolyte measured using an X-ray diffraction device employing CuKα radiation, the value of IA / IB is 1.0 or more, and the value of IC / ID is less than 0.90, where IA is the integrated intensity of a diffraction peak observed at a position of 2θ=15.4°±1°, IB is the integrated intensity of a diffraction peak observed at a position of 2θ=17.8°±1°, IC is the integrated intensity of a diffraction peak observed at a position of 2θ=25.3°±1°, and ID is the integrated intensity of a diffraction peak observed at a position of 2θ=29.7°±1°.

Description

Solid electrolyte, electrode mixture containing the same, solid electrolyte layer and solid battery, and method for evaluating the solid electrolyte

[0001] The present invention relates to a solid electrolyte. The present invention also relates to an electrode mixture containing a solid electrolyte, a solid electrolyte layer, and a solid-state battery. The present invention further relates to a method for evaluating a solid electrolyte.

[0002] In recent years, secondary batteries have been attracting attention as an effort to prevent global warming by reducing carbon dioxide emissions. Among these, solid-state batteries using sulfide solid electrolytes have attracted attention. Solid-state batteries using sulfide solid electrolytes have the advantage of simplifying safety devices and achieving excellent manufacturing costs and productivity because they do not use flammable organic solvents. Furthermore, this type of solid electrolyte is advantageous from the perspective of improving safety and durability, since ionic species other than lithium ions do not migrate within the electrolyte, preventing side reactions due to the migration of anions.

[0003] In order to improve the performance of solid-state batteries, the present applicant has previously developed a crystalline phase having a cubic argyrodite-type crystal structure, ７－ｘ P.S. ６－ｘ Cl ｙ Br ｚ (See Patent Document 1.) This sulfide solid electrolyte can achieve low elasticity while maintaining high ionic conductivity, and therefore, by using this sulfide solid electrolyte as a material for a solid-state battery, there is an advantage that the resistance of the solid-state battery can be reduced.

[0004] US2020 / 127325A1

[0005] The present inventors have conducted extensive research to further improve the performance of solid-state batteries and have found a safety issue in that an unintended change in the environment during use of a solid-state battery, such as an increase in the ambient temperature, can cause problems such as thermal runaway in the solid-state battery. In other words, an object of the present invention is to provide a solid electrolyte that enables a solid-state battery to be used safely even when the environment in which it is used changes.

[0006] In the present invention, the integrated intensity of the diffraction peak observed at 2θ=15.4°±1° in an X-ray diffraction pattern measured by an X-ray diffractometer using CuKα radiation is defined as I Ａ The integrated intensity of the diffraction peak observed at 2θ = 17.8° ± 1° is defined as I Ｂ The integrated intensity of the diffraction peak observed at 2θ = 25.3° ± 1° is defined as I Ｃ The integrated intensity of the diffraction peak observed at 2θ = 29.7° ± 1° is defined as I Ｄ When this is done, I Ａ / I Ｂ The value of is 1.0 or more, and I Ｃ / I Ｄ The present invention provides a solid electrolyte having a value of less than 0.90.

[0007] In addition, the present invention is directed to a method for determining the maximum peak intensity of 2471 eV±1 eV in the K-absorption edge spectrum of sulfur element obtained by the total electron yield method of X-ray absorption fine structure analysis. Ｘ The maximum peak intensity of 2475 eV ± 2 eV is I Ｙ When I Ｘ / I Ｙ The present invention provides a solid electrolyte having a value of 0.89 or less.

[0008] The present invention also provides a method for evaluating a solid electrolyte, which comprises mixing a transition metal oxide and a solid electrolyte, performing differential thermal analysis, and evaluating the oxidation resistance of the solid electrolyte based on the temperature at which an exothermic peak occurs.

[0009] Figure 1 shows X-ray diffraction charts (2θ = 10 to 40°) of the solid electrolytes obtained in the examples and comparative examples. Figure 2 shows K-edge absorption spectra of sulfur element obtained by measuring the solid electrolytes obtained in the examples and comparative examples by the total electron yield method of X-ray absorption fine structure analysis.

[0010] The present invention will be described below based on preferred embodiments. The present invention relates to a solid electrolyte. The solid electrolyte of the present invention preferably has lithium ion conductivity. As long as it has lithium ion conductivity, the type and composition of the solid electrolyte are not particularly limited, and an appropriate one can be selected appropriately depending on the application, desired physical properties, etc. In particular, the solid electrolyte is preferably a sulfide solid electrolyte. The sulfide solid electrolyte preferably contains lithium (Li), phosphorus (P), sulfur (S), and a halogen (X). Examples of the halogen (X) include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). The halogen (X) may be one of these elements or a combination of two or more of them. In particular, the halogen (X) is preferably a combination of chlorine (Cl) and bromine (Br).

[0011] Such solid electrolytes include, for example, Li ２ S-P ２ S ５ -LiX (where "X" represents one or more halogen elements), Li ａ P.S. ｂ X ｃ (a solid electrolyte having a crystalline phase of an argyrodite-type crystal structure, "X" represents one or more halogen elements.)

[0012] The solid electrolyte of the present invention is preferably a crystalline substance. The solid electrolyte of the present invention preferably exhibits a diffraction peak at a specific angle in a diffraction pattern obtained by subjecting the solid electrolyte to X-ray diffraction. The solid electrolyte of the present invention exhibits a diffraction peak at a specific angle, and has the advantageous effect of being less likely to react with the active material, thereby enabling safe use even when the use environment of a solid battery containing the solid electrolyte becomes unintentionally severe, for example, when the temperature becomes high.

[0013] Specifically, in the solid electrolyte of the present invention, in an X-ray diffraction pattern measured using an X-ray diffractometer, diffraction peak A is preferably observed at 2θ = 15.4° ± 1°, diffraction peak B is preferably observed at 2θ = 17.8° ± 1°, diffraction peak C is preferably observed at 2θ = 25.3° ± 1°, and diffraction peak D is preferably observed at 2θ = 29.7° ± 1°. The range of each diffraction peak may be ±0.7°, ±0.5°, or ±0.3°. When obtaining the X-ray diffraction pattern, Cu-Kα is used as the radiation source. Hereinafter, in all references to X-ray diffraction patterns in this specification, Cu-Kα is used as the radiation source.

[0014] In the solid electrolyte of the present invention, in addition to the diffraction peaks being observed at the angles mentioned above, it is preferable that the intensity ratio of two specific diffraction peaks has a specific relationship. This further provides the advantageous effect that even if the use environment of a solid battery containing the solid electrolyte of the present invention becomes unintentionally severe, for example, even if the temperature becomes high, the reaction with the active material is unlikely to occur, and the battery can be used safely. In particular, the reaction between the solid electrolyte of the present invention and the active material is suppressed. The reason for this is not clear, but the inventors believe it to be as follows. It is known that argyrodite-type solid electrolytes (space group: F-43m) have anion sites at two Wyckoff positions, 4a and 4d. Here, the integrated intensity of diffraction peak A is expressed as I Ａ and the integrated intensity of the diffraction peak B is I Ｂ , the integrated intensity of the diffraction peak C is I Ｃ and the integrated intensity of the diffraction peak D is I Ｄ When I Ａ / I Ｂ and I Ｃ / I Ｄ The inventors have found that I is closely related to the electron density at the 4d site. Specifically, the amount of light elements present at the 4d site increases, and I Ａ / I Ｂ increases, and I Ｃ / I ＤThe 4d site is surrounded by the cation Li site (24g, 48h), so the proportion of ions with low electron density in the 4d site increases, and Li ＋ It is believed that this increases the electrostatic attraction between the 4d sites and stabilizes the structure. In the present invention, the above-mentioned problem has been solved by increasing the proportion of light elements in the constituent elements and by appropriately controlling the manufacturing conditions to increase the proportion of light elements occupying the 4d sites.

[0015] From the above viewpoint, the integrated intensity of the diffraction peak A is expressed as I Ａ and the integrated intensity of the diffraction peak B is I Ｂ When I Ａ / I Ｂ The value of I is preferably 1.0 or more, more preferably 1.2 or more, and even more preferably 1.3 or more. Ａ / I Ｂ The value of is preferably 3 or less, more preferably 2.5 or less, even more preferably 2.0 or less, and even more preferably 1.5 or less.

[0016] In the solid electrolyte of the present invention, the integrated intensity of the diffraction peak C is I Ｃ and the integrated intensity of the diffraction peak D is I Ｄ When I Ｃ / I Ｄ The value of I is preferably smaller than 0.90, and more preferably 0.89 or less. Ｃ / I Ｄ The value of is preferably 0.6 or more, more preferably 0.7 or more, even more preferably 0.75 or more, even more preferably 0.8 or more, and particularly preferably 0.85 or more.

[0017] As described above, the solid electrolyte of the present invention is preferably a crystalline substance, and in particular, the solid electrolyte preferably has an argyrodite-type crystal structure because it has excellent lithium ion conductivity. When the solid electrolyte of the present invention has a crystalline phase with an argyrodite-type crystal structure, the solid electrolyte has a composition formula (I): Li ａ P.S. ｂ X ｃ (X is at least one of fluorine (F), chlorine (Cl), bromine (Br), and iodine (I)) is preferred from the viewpoint of improving lithium ion conductivity.

[0018] When the solid electrolyte of the present invention has a crystalline phase of an argyrodite-type crystal structure, the above-mentioned diffraction peak A corresponds to the 111 plane in the crystal structure. Diffraction peak B corresponds to the 020 plane in the argyrodite-type crystal structure. Diffraction peak C corresponds to the 220 plane in the argyrodite-type crystal structure. Diffraction peak D corresponds to the 311 plane in the argyrodite-type crystal structure.

[0019] The solid electrolyte of the present invention has the above-mentioned intensity ratio I Ａ / I Ｂ and I Ｃ / I Ｄ In order to achieve this, the inventors have found that it is advantageous to adjust the molar ratio of the X element to the P element (hereinafter also referred to as "X / P") in the composition formula (I) to a relatively low value. In detail, when the molar ratio X / P is made smaller than 1.2, the intensity ratio I Ａ / I Ｂ and I Ｃ / I Ｄ From this viewpoint, the molar ratio X / P is preferably smaller than 1.0, and more preferably smaller than 0.9. The molar ratio X / P may be 0.1 or more, or 0.3 or more, or even 0.6 or more. When the molar ratio X / P is in this range, the monovalent anion X decreases, and the divalent anion S occupies more of the overall composition. ２－This increases the electrostatic energy between the solid electrolyte and lithium ions, improving the lattice energy of the solid electrolyte, which is advantageous in that it can suppress the reactivity between the solid electrolyte and the active material.

[0020] The solid electrolyte of the present invention has the above-mentioned intensity ratio I Ａ / I Ｂ and I Ｃ / I Ｄ In order to achieve this, the inventors have found that it is also advantageous to adjust the molar ratio of S element to P element (hereinafter also referred to as "S / P") in the composition formula (I). In particular, when the molar ratio S / P is made larger than 4.8, the above-mentioned intensity ratio I Ａ / I Ｂ and I Ｃ / I Ｄ is easily achieved, which is preferable. From this viewpoint, the molar ratio S / P is preferably 5.0 or more, and more preferably 5.1 or more. The molar ratio S / P may be 6.8 or less, or 6.0 or less, or 5.6 or less, or 5.3 or less. When the molar ratio S / P is in this range, the electrostatic energy between the solid electrolyte and lithium ions increases while maintaining high ionic conductivity, and the lattice energy of the solid electrolyte is improved, which is preferable in that the reactivity between the solid electrolyte and the active material can be suppressed.

[0021] Furthermore, in the solid electrolyte of the present invention, the molar ratio of Li to P (hereinafter also referred to as "Li / P") is preferably, for example, 4.0 or more, more preferably 5.0 or more, and particularly preferably 6.0 or more. On the other hand, the molar ratio Li / P is preferably, for example, 7.0 or less, more preferably 6.8 or less, and particularly preferably 6.4 or less. When the molar ratio Li / P is within this range, the argyrodite-type crystal structure, particularly the cubic argyrodite-type crystal structure, becomes more stable at around room temperature (25°C), allowing lithium ion vacancies to be sufficiently introduced into the structure, resulting in effectively increasing lithium ion conductivity.

[0022] The argyrodite-type crystal phase is represented by the following formula (II):７－ｄ P.S. ６－ｄ X ｄ ...(II) The composition represented by formula (II) is the stoichiometric composition of an argyrodite-type crystal phase.

[0023] In the sulfide solid electrolyte of the present invention, from the viewpoint of achieving both high ionic conductivity and difficulty in reaction between the solid electrolyte and the active material, d is preferably 0.1 or more and less than 1.2, more preferably 0.3 or more and 1.0 or less, and even more preferably 0.6 or more and 0.9 or less.

[0024] In formula (II), a part of P may be substituted with one or more elements selected from silicon (Si), germanium (Ge), tin (Sn), lead (Pb), boron (B), aluminum (Al), gallium (Ga), arsenic (As), antimony (Sb), and bismuth (Bi).

[0025] The ratio of each element contained in the solid electrolyte can be measured using, for example, inductively coupled plasma (ICP) emission spectroscopy, energy dispersive X-ray spectroscopy (EDS), or X-ray fluorescence analysis (XRF).

[0026] While the crystallinity of solid electrolytes generally decreases as the particle size decreases, the solid electrolyte of the present invention preferably has a small particle size and high crystallinity. High crystallinity means that the solid electrolyte has few reactive active sites. Therefore, even if the usage environment of a solid battery containing the solid electrolyte of the present invention becomes unintentionally harsh, for example, even at high temperatures, reaction with the active material is unlikely to occur. The crystallinity of the solid electrolyte can be evaluated by the half-width of the X-ray diffraction peak. The smaller the half-width value, the higher the crystallinity of the solid electrolyte can be evaluated. In the solid electrolyte of the present invention, the half-width of diffraction peak D is preferably 0.16° or less, more preferably 0.15° or less, and even more preferably 0.14° or less. The reason for selecting diffraction peak D for evaluating the half-width is that diffraction peak D is the most intense peak in the solid electrolyte of the present invention, making it easy to calculate the half-width.

[0027] The crystallinity of the solid electrolyte of the present invention can also be evaluated by the crystallite size. Specifically, the crystallite size can be calculated based on the X-ray diffraction pattern measured for the solid electrolyte of the present invention. The solid electrolyte of the present invention preferably has a crystallite size of 450 Å or more, more preferably 550 Å or more, and even more preferably 600 Å or more. There is no particular limit to the upper limit of the crystallite size, and it may be, for example, 2000 Å or less, 1000 Å or less, or 700 Å or less. The method for measuring the crystallite size will be explained in the Examples below.

[0028] The crystallinity of the solid electrolyte of the present invention can also be evaluated by the ratio of the particle size of the solid electrolyte to the crystallite size. Specifically, the volume cumulative particle size at 50% cumulative volume measured by a laser diffraction / scattering particle size distribution measurement method is expressed as D ５０ (μm) and the crystallite size is C (Å), (D ５０ x10 ４ The value of (D) / C is preferably 2 or more, more preferably 6 or more, and even more preferably 7 or more. ５０ x10 ４ The value of (D) / C is preferably 15 or less, more preferably 12 or less, even more preferably 10 or less, even more preferably 9 or less, and particularly preferably 8 or less. ５０ x10 ４ The closer the value of ) / C is to 1, the closer the particles are to a single crystal state, and reflects the degree of crystallinity relative to the particle size.

[0029] (D ５０ x10 ４ ) / C is as described above, and the particle size D ５０ The value of the particle diameter itself is preferably less than 5 μm, more preferably 2 μm or less, even more preferably 1 μm or less, and even more preferably 0.8 μm or less. By having such a particle diameter, the contact points and contact area between the solid electrolyte and the active material particles become large, and the input / output characteristics of the battery can be effectively improved. ５０There is no particular limitation on the lower limit of the particle size, and it may be, for example, 0.1 μm or more, 0.3 μm or more, or 0.5 μm or more. By having such a particle size, an excessive increase in the surface area of ​​the solid electrolyte can be suppressed, and an increase in resistance can be suppressed. In addition, it becomes easy to mix with the active material.

[0030] The solid electrolyte of the present invention also has a characteristic feature in the K-edge absorption spectrum of sulfur element obtained by measuring it using the total electron yield method of the X-ray absorption fine structure (hereinafter also referred to as "XAFS"). XAFS is a technique for analyzing absorption spectra obtained by irradiating a material with X-rays. In the spectrum obtained by irradiating a material with X-rays, a rise in absorption intensity called an absorption edge is observed at the incident X-ray energy specific to the element contained in the material. The structure that appears around ±50 eV near this absorption edge is called X-ray absorption near edge structure (XANES), and it enables evaluation of the chemical state (valence, coordination structure) of the element of interest in the sample. However, the X-ray transmittance of the sample is extremely low in the incident X-ray energy range of 4000 eV or less, which includes many absorption edges of the main components contained in the solid electrolyte of the present invention. Therefore, it is difficult to obtain a spectrum using the transmission method, which measures the X-ray intensity before and after incidence on the sample. In this case, the total electron yield (TEY) method is effective, which utilizes the electrons emitted from a sample upon X-ray irradiation and measures the sample current to determine the X-ray absorption intensity. By XAFS measurement using the TEY method, the solid electrolyte of the present invention can be used as the measurement target, and the chemical state of sulfur can be obtained element-selectively.

[0031] In the solid electrolyte of the present invention, the maximum peak intensity observed at 2471 eV±1 eV in the K absorption edge spectrum of the sulfur element is I Ｘ The maximum peak intensity observed at 2475 eV±2 eV is defined as I Ｙ When I Ｘ / I ＹThe ratio (hereinafter also referred to as "XAFS intensity ratio") is preferably 0.89 or less. When the solid electrolyte has such an XAFS intensity ratio, even if the use environment of a solid battery containing the solid electrolyte becomes unintentionally severe, the reaction with the active material is unlikely to occur, and the advantageous effect of safe use is further enhanced. The reason for this is not clear, but the present inventors hypothesize as follows: The peak observed at 2471 eV±1 eV is due to PS in the solid electrolyte. ４ ３－ This is a peak derived from the S unit. The S that does not bond with P accounts for the entire S element in the solid electrolyte. ２－ As the proportion of increases, I Ｘ / I Ｙ It is thought that this will decrease Li ＋ The electrostatic attraction between the active material and the solid electrolyte of the present invention is increased, thereby stabilizing the structure and making it less likely to react with the active material. In particular, the solid electrolyte of the present invention is less likely to react with the active material and the ionic conductivity of the solid electrolyte is improved at the same time. From the viewpoint of making this advantage even more pronounced, the XAFS intensity ratio I Ｘ / I Ｙ It is more preferable that the XAFS intensity ratio I be 0.88 or less. Ｘ / I Ｙ The method for measuring the K-absorption edge spectrum of elemental sulfur by the total electron yield method of the XAFS method will be described in the examples below.

[0032] The solid electrolyte of the present invention has lithium ion conductivity in a solid state. The solid electrolyte preferably has a lithium ion conductivity of 0.5 mS / cm or more, and more preferably 1.0 mS / cm or more, at room temperature, i.e., 25° C. The lithium ion conductivity can be measured using the method described in the Examples below.

[0033] Next, a preferred method for producing the solid electrolyte of the present invention will be described. The solid electrolyte can be preferably produced by a solid-phase reaction in which a raw material composition is heated and sintered. The raw material composition is a mixture of raw material powders containing the above-mentioned elements that constitute the solid electrolyte. The raw material composition contains one or more compounds containing at least one of Li, P, S, and X.

[0034] The raw material powder may be, for example, a compound containing an Li element, a compound containing an S element, a compound containing a P element, or a compound containing an X element.

[0035] The raw material powder may contain at least two or more elements selected from Li, P, S, and X in one compound. For example, the raw material powder may be a compound containing Li and X, a compound containing P and S, a compound containing Li and S, a compound containing P and X, or a compound containing S and X. An example of the compound containing Li and X is lithium halide. An example of the compound containing P and S is diphosphorus trisulfide (P ２ S ３ ) and diphosphorus pentasulfide (P ２ S ５ As a compound containing Li and S, for example, lithium sulfide (Li ２ As a compound containing P element and X element, for example, PX ３ and P ２ X ５ Examples of compounds containing S and X elements include SX ２ , SX ４ , SX ６ , S ２ X １０ Sulfur halides such as the following can be used.

[0036] The raw material powders are preferably subjected to a pulverization process before being mixed to adjust the particle size to a predetermined size. For pulverization, a media-agitating mill such as a ball mill or a bead mill can be used. When pulverization is performed using a media-agitating mill, a slurry of the raw material powder is placed in a container, and ceramic or metal balls or beads are placed in the container. The container is rotated to cause the raw material powder to collide with the balls or beads in the container, thereby pulverizing the raw material powder. Although pulverization tends to reduce the crystallinity of the raw material powder, in this production method, it is preferable to perform pulverization so as to minimize the reduction in the crystallinity of the raw material powder to be pulverized. By performing such pulverization, a solid electrolyte with a small particle size and high crystallinity can be obtained, and as a result, a solid electrolyte satisfying the above-mentioned X-ray intensity ratio can be obtained.

[0037] After the grinding of the raw material powders is completed, the raw material powders are mixed to obtain a raw material composition. The raw material powders are preferably mixed so that the molar ratio X / P in the target solid electrolyte is smaller than 1.2, since this facilitates the production of a solid electrolyte that satisfies the above-mentioned strength ratio. From the same viewpoint, the raw material powders are preferably mixed so that the molar ratio S / P in the target solid electrolyte is larger than 4.8.

[0038] It is also preferable to use a media stirring mill for mixing the raw material powders. In this case, it is preferable to mix the raw material powders so as not to reduce the crystallinity of the raw material powders as much as possible. By mixing in this manner, a solid electrolyte with a small particle size and high crystallinity can be obtained, and as a result, a solid electrolyte satisfying the above-mentioned strength ratio can be obtained.

[0039] Next, the raw material composition is subjected to a calcination process to cause a solid-state reaction and obtain a crystalline calcined product. The calcination atmosphere can be, for example, an inert gas atmosphere such as an argon atmosphere or a nitrogen atmosphere, or a hydrogen sulfide atmosphere. From the viewpoint of adjusting the ratio of sulfur element contained in the solid electrolyte, it is preferable to use an inert gas atmosphere.

[0040] From the viewpoint of ensuring that a solid-phase reaction of the raw material composition occurs, the firing temperature is, for example, preferably 200° C. or higher, more preferably 300° C. or higher, even more preferably 350° C. or higher, and even more preferably 400° C. or higher. On the other hand, in consideration of industrial producibility and economic efficiency, the firing temperature is, for example, preferably 700° C. or lower, more preferably 600° C. or lower, and even more preferably 550° C. or lower.

[0041] The firing time is not critical, and may be any time that allows a fired product of the desired composition to be obtained. Specifically, the firing time is preferably long enough for the solid-phase reaction of the raw material composition to occur sufficiently. The firing time may be, for example, 30 minutes or more, 2 hours or more, or 3 hours or more. On the other hand, the firing time may be, for example, 10 hours or less, or 5 hours or less.

[0042] The solid electrolyte thus obtained can be used alone or in combination with other solid electrolytes. For example, the solid electrolyte of the present invention can be used as a material for constituting a lithium battery, such as a solid electrolyte layer, a positive electrode layer, or a negative electrode layer.

[0043] Specifically, the solid electrolyte of the present invention can be used in a battery having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer located between the positive electrode layer and the negative electrode layer. In other words, the solid electrolyte can be used in so-called solid-state batteries. More specifically, it can be used in lithium solid-state batteries. The lithium solid-state battery may be a primary battery or a secondary battery. There is no particular limitation on the shape of the battery, and shapes such as a laminated type, a cylindrical type, and a prismatic type can be adopted. The term "solid-state battery" includes not only solid-state batteries that do not contain any liquid or gel-like substance as an electrolyte, but also batteries that contain, for example, 50% by mass or less, 30% by mass or less, or 10% by mass or less of a liquid or gel-like substance as an electrolyte.

[0044] When the solid electrolyte layer contains the solid electrolyte of the present invention, the solid electrolyte layer can be produced, for example, by dropping a slurry consisting of the solid electrolyte, a binder, and a solvent onto a substrate and scraping it off with a doctor blade or the like, by contacting the substrate with the slurry and then cutting it with an air knife, or by forming a coating film by screen printing or the like and then heating and drying to remove the solvent. Alternatively, the solid electrolyte can be produced by compacting a powdered solid electrolyte by pressing or the like and then appropriately processing it. The thickness of the solid electrolyte layer is typically preferably 5 μm to 300 μm, and more preferably 10 μm to 100 μm, in order to balance short-circuit prevention and volumetric capacity density.

[0045] The solid electrolyte of the present invention can also be used together with an active material to form an electrode mixture. The proportion of the solid electrolyte in the electrode mixture is typically 10% by mass or more and 50% by mass or less. The electrode mixture may contain other materials such as a conductive material as needed. An electrode mixture, a binder, and a solvent are mixed to form a paste, which is then applied to a current collector such as aluminum foil and dried to form electrodes such as a positive electrode and a negative electrode.

[0046] The cathode material constituting the cathode layer can be any cathode material used as a cathode active material in lithium-ion batteries. For example, lithium-containing cathode active materials, specifically spinel-type lithium transition metal oxides and lithium metal oxides with layered structures, can be used. The use of a high-voltage cathode material as the cathode material can improve energy density. In addition to the cathode active material, the cathode material may contain a conductive material or other materials.

[0047] As the negative electrode material constituting the negative electrode layer, a negative electrode material used as a negative electrode active material in a lithium ion battery can be appropriately used. Since the solid electrolyte of the present invention is electrochemically stable, it can be used at a potential of lithium metal or a lower potential comparable to that of lithium metal (about 0.1 V vs. Li ＋Carbonaceous materials such as graphite, artificial graphite, natural graphite, and non-graphitizable carbon (hard carbon), which are materials that are charged and discharged using a lithium-ion battery (Li / Li), can be used as the negative electrode material. This can significantly improve the energy density of solid-state batteries. Silicon or tin, which are promising high-capacity materials, can also be used as the active material. The negative electrode material may also contain a conductive material or other materials in addition to the negative electrode active material.

[0048] The present invention also provides a method for evaluating a solid electrolyte. This method relates to evaluating the oxidation resistance of a solid electrolyte. If an unintended environmental change occurs during use of a solid-state battery, such as an increase in ambient temperature, the solid electrolyte may react with oxygen contained in the active material, causing a malfunction in the solid-state battery. Therefore, it is desirable to evaluate the oxidation resistance of the solid electrolyte before incorporating it into a solid-state battery. The evaluation method of the present invention is extremely useful from this perspective.

[0049] In this evaluation method, a transition metal oxide and a solid electrolyte are mixed, and differential thermal analysis is performed to evaluate the oxidation resistance of the solid electrolyte based on the temperature at which an exothermic peak occurs. The transition metal oxide simulates a positive electrode in a charged state in which lithium ions have been desorbed from a spinel-type lithium manganate or a rock salt layered positive electrode active material. From this perspective, the transition metal in the transition metal oxide is preferably an element used as a positive electrode active material in lithium solid-state batteries. Examples of such transition metals include manganese (Mn) and nickel (Ni). Examples of oxides containing the transition metal include MnO, an oxide of Mn. ２ , Mn ３ O ４ , Mn ２ O ３ , MnO and Mn ２ O ７ In addition, NiO, which is an oxide of Ni, can be mentioned. ２ and NiO. These transition metal oxides may be used alone or in combination of two or more.

[0050] Differential thermal analysis is performed on a mixture of a solid electrolyte and a transition metal oxide. From the viewpoint of improving the accuracy of the evaluation, the mass ratio of the solid electrolyte to the transition metal oxide in this mixture is preferably, for example, 1% by mass or more and 90% by mass or less, more preferably 5% by mass or more and 50% by mass or less, where the mass of the solid electrolyte is relative to the total mass of the solid electrolyte and the transition metal oxide. Furthermore, the mass ratio of the transition metal oxide to the total mass of the solid electrolyte and the transition metal oxide is preferably 10% by mass or more and 99% by mass or less, more preferably 50% by mass or more and 95% by mass or less. The mixture of the solid electrolyte and the transition metal oxide may contain only these two substances, or may contain a substance commonly blended in lithium solid-state batteries as a third component. However, from the viewpoint of further improving the accuracy of the evaluation, the mixture preferably contains only the solid electrolyte and the transition metal oxide.

[0051] It is preferable to perform differential thermal analysis measurement in an inert gas atmosphere from the viewpoint of improving measurement accuracy. Examples of inert gas include nitrogen and rare gases such as argon and helium. A temperature range of 25°C or higher and 500°C or lower for differential thermal analysis measurement is sufficient for evaluating the oxidation resistance of a solid electrolyte. However, the upper limit of the measurement temperature may be higher than 500°C or lower than 500°C. From the viewpoint of improving measurement accuracy, it is preferable to set the temperature rise rate of differential thermal analysis measurement to 1°C / min or higher and 50°C / min or lower.

[0052] When a mixture containing a solid electrolyte and a transition metal oxide is subjected to differential thermal analysis, an exothermic peak is observed at a predetermined temperature. This exothermic peak is observed due to oxidation of the solid electrolyte. The higher the temperature at which the exothermic peak is observed, the higher the oxidation resistance of the solid electrolyte can be evaluated.

[0053] In this evaluation method, mass spectrometry may be performed simultaneously with differential thermal analysis. Mass spectrometry can evaluate the type and amount of gas generated from the solid electrolyte during the heating process. Examples of gases generated from the solid electrolyte include sulfur oxides, sulfur, and hydrogen sulfide. In this evaluation method, evaluating the type of gas generated is useful because it allows for understanding the harmfulness to humans and the ignition temperature of the gas generated by the reaction between the solid electrolyte and the active material. Meanwhile, evaluating the amount of gas generated is useful because it allows for understanding the explosive limit concentration of the gas generated when the solid electrolyte reacts with the active material and can serve as a guide for the rate of internal pressure increase within the cell. To simultaneously perform differential thermal analysis and mass spectrometry, for example, a TG-DTA-MS device may be used.

[0054] In relation to the above-described embodiments, the present invention further discloses the following solid electrolyte, electrode mixture, electrode layer, solid battery, and method for evaluating the solid electrolyte: [1] In an X-ray diffraction pattern measured by an X-ray diffractometer using CuKα radiation, the integrated intensity of a diffraction peak observed at 2θ=15.4°±1° is defined as I Ａ The integrated intensity of the diffraction peak observed at 2θ = 17.8° ± 1° is defined as I Ｂ The integrated intensity of the diffraction peak observed at 2θ = 25.3° ± 1° is defined as I Ｃ The integrated intensity of the diffraction peak observed at 2θ = 29.7° ± 1° is defined as I Ｄ When this is done, I Ａ / I Ｂ The value of is 1.0 or more, and I Ｃ / I Ｄ [2] A solid electrolyte having a maximum peak intensity of 2471 eV±1 eV in the K-edge spectrum of sulfur element obtained by the total electron yield method of X-ray absorption fine structure analysis. Ｘ The maximum peak intensity of 2475 eV ± 2 eV is I Ｙ When I Ｘ / I Ｙ[3] The solid electrolyte according to [1] or [2], which contains lithium (Li), phosphorus (P), sulfur (S), and halogen (X), and wherein the molar ratio of the halogen (X) to the phosphorus (P) is less than 1.2. [4] The solid electrolyte according to [3], which has a molar ratio of the sulfur (S) to the phosphorus (P) element greater than 4.8. [5] The solid electrolyte according to [4], which has a volume cumulative particle size D at 50% of the cumulative volume measured by a laser diffraction / scattering particle size distribution measurement method. ５０ [6] The solid electrolyte according to any one of [1] to [4], wherein the particle diameter D relative to the crystallite size (Å) determined based on the X-ray diffraction pattern is less than 5 μm, and the half width of a diffraction peak observed at 2θ=29.7°±1° in the X-ray diffraction pattern is 0.16° or less. ５０ (μm) 10 ４ [5] The solid electrolyte according to [5], wherein the ratio of is 1 or more and 15 or less. [7] The solid electrolyte according to any one of [1] to [6], which contains a crystalline phase having an argyrodite-type crystal structure. [8] An electrode mixture comprising the solid electrolyte according to any one of [1] to [7] and an active material. [9] A solid electrolyte layer containing the solid electrolyte according to any one of [1] to [7].

[10] A solid battery having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer located between the positive electrode layer and the negative electrode layer, the solid electrolyte containing the solid electrolyte according to any one of [1] to [7].

[11] A method for evaluating a solid electrolyte, comprising mixing a transition metal oxide and a solid electrolyte, performing differential thermal analysis, and evaluating the oxidation resistance of the solid electrolyte based on the temperature at which an exothermic peak occurs.

[12] The transition metal oxide is MnO ２ , Mn ３ O ４ , Mn ２ O ３ , MnO, Mn ２ O ７ , NiO ２

[13] The evaluation method according to

[11] or

[12] , wherein mass spectrometry is performed simultaneously with the differential thermal analysis measurement to evaluate the type and amount of gas generated from the solid electrolyte.

[0055] The present invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited to these examples.

[0056] Example 1 Lithium sulfide (Li ２ S) powder, diphosphorus pentasulfide (P ２ S ５ Lithium chloride (LiCl) powder, lithium bromide (LiBr) powder, and lithium fluoride (LiBr) powder were used as raw material powders. Toluene was added to each raw material powder to prepare a slurry. Each slurry was individually placed in a polyamide container and set in a Fritsch planetary ball mill P-5. Zirconia balls with a diameter of 5 mm were used as the grinding media. The ball mill was operated at 100 rpm and wet-milled for 10 hours. The toluene was removed from the ground slurry by vacuum drying at 150°C. Each ground raw material powder was obtained in this manner. Each raw material powder was weighed and mixed to obtain the composition shown in Table 1 below, and toluene was added to prepare a slurry. Each slurry was individually placed in a polyamide container and set in the planetary ball mill. Zirconia balls with a diameter of 5 mm were used as the grinding media. The ball mill was operated at 100 rpm and wet-milled for 10 hours. The toluene was removed from the mixed slurry by vacuum drying at 80°C. A raw material composition was thus obtained.

[0057] The raw material composition was fired to obtain a fired product. The firing was carried out using a tubular electric furnace. Nitrogen gas with a purity of 100% was circulated through the electric furnace during firing. The firing was carried out by raising the temperature to 500°C at 200°C / h, maintaining the temperature at 500°C for 4 hours, and then allowing it to cool to room temperature. In this way, a fired product was obtained.

[0058] The obtained fired product was crushed in a mortar and passed through a 250 μm sieve to obtain a crushed powder. This crushed powder was crushed using a planetary ball mill. Zirconia balls with a diameter of 5 mm were used as the crushing media. The container was made of polyamide. Toluene was used as the solvent. The ball mill was operated at 100 rpm, and crushing was carried out for 3 hours. The obtained slurry was vacuum dried at 80°C to remove the solvent. In this way, a crushed powder was obtained.

[0059] The obtained pulverized powder was further pulverized in a planetary ball mill. Zirconia balls with a diameter of 0.8 mm were used as the pulverization media. The container was made of polyamide. Toluene containing a dispersant was used as the solvent. The ball mill was operated at 100 rpm, and pulverization was continued until the desired particle size was achieved. The obtained slurry was vacuum dried at 80°C to remove the solvent. In this way, the desired solid electrolyte was obtained.

[0060] [Examples 2 and 3] Li was added to obtain the composition shown in Table 1 below. ２ S powder and P ２ S ５ The powder, LiCl powder, and LiBr powder were weighed separately to obtain a raw material composition. Except for this, the same procedure as in Example 1 was carried out to obtain a powder of a solid electrolyte.

[0061] [Comparative Examples 1 and 2] Li was added to obtain the composition shown in Table 1 below. ２ S powder and P ２ S ５ The powder, LiCl powder, and LiBr powder were weighed to obtain a raw material composition. Except for this, the same procedure as in Example 1 was carried out to obtain a powder of a solid electrolyte.

[0062] [Evaluation] The solid electrolytes obtained in the examples and comparative examples were subjected to X-ray diffraction measurement by the following method, and the intensity ratio I Ａ / I Ｂ and I Ｃ / I Ｄ The value of was determined. The half-width of the diffraction peak D was also determined. Furthermore, the crystallite size was determined. X-ray diffraction charts for the solid electrolytes of the Examples and Comparative Examples are shown in Figure 1. Furthermore, the K-absorption edge spectrum of the sulfur element was measured by the total electron yield method of the XAFS method in the following manner. The spectra (except for Example 3) are shown in Figure 2. Furthermore, the particle diameter D ５０ The conductivity and reaction onset temperature with manganese dioxide were measured, and the results are shown in Table 1 below.

[0063] [X-ray diffraction measurement] Measurements were performed using a Malvern Panalytical tabletop X-ray diffractometer "Aeris" without exposure to air. The measurement conditions were as follows: - Radiation source: CuKα - Tube voltage: 40 kV - Tube current: 15 mA - Measurement method: Focusing method (reflection method) - Detector: One-dimensional semiconductor detector - Incident Soller slit: Soller slit 0.02 rad - Longitudinal limiting slit: 20 mm - Receiving Soller slit: 0.02 rad - Incident slit: 1 / 2° - Receiving slit: Open - Measurement range: 2θ = 10 to 105° - Step width: 0.01° - Scan speed: 1.67° / min. The background intensity obtained from the Kapton film in the non-exposed holder was subtracted before analyzing the measurement results.

[0064] [Integrated intensity, half-width and crystallite size] The X-ray diffraction pattern obtained by X-ray diffraction measurement was read into Smart Lab Studio II and calculated by peak processing. Peak profiling was performed using a peak shape: divided pseudo-Voight function, background type: B-spline, and fitting conditions: automatic. From the obtained peak list results, the integrated intensity (Count °) and half-width FWHM (°) of the corresponding peak were read. The crystallite size (Å) was calculated based on the peak at 2θ = 29.7 ° ± 1 °.

[0065] [K-edge absorption spectrum of elemental sulfur] The K-edge absorption spectrum of elemental sulfur was measured by the total electron yield method of the XAFS method according to the following method. ・Experimental facility: Ritsumeikan University SR Center ・Experimental beamline: BL-10 ・Spectrometer: Ge (111) double crystal spectrometer ・Incident X-ray size: 2 mm length x 5 mm width ・Measurement method: Total electron yield (TEY) method ・Measurement energy range: 2400 to 2850 eV For TEY measurements, carbon tape is attached to a SUS sample holder, and the measurement sample is applied onto it. A transfer vessel is used to transport the measurement sample, and it is introduced into the beamline without being exposed to the atmosphere. At each incident X-ray energy (E, x-axis), the incident X-ray intensity (I ０ ) and specimen current (I ｅ) is measured, and the X-ray absorption intensity (y-axis) is calculated using the following formula. The XAFS spectrum is obtained by plotting the X-ray absorption intensity μt=I ｅ / I ０ The analysis software "Athena" (Demeter ver. 0.9.26) is used for background processing and normalization of the data obtained as described above. After reading the XAFS spectrum with this software, the pre-edge region (region from approximately -70 eV to -30 eV based on the absorption edge) and the post-edge region (region from approximately -150 eV to -310 eV based on the absorption edge) are fitted to subtract the background absorption, and the XAFS spectrum is normalized so that the absorbance before and after the absorption edge becomes 1. The XAFS intensity ratio is calculated from the normalized XAFS spectrum as I. Ｘ / I Ｙ Ask for.

[0066] [Particle size D ５０ The solid electrolyte was added to toluene using an automatic sample feeder for a laser diffraction particle size distribution analyzer ("Microtorac SDC" manufactured by Nikkiso Co., Ltd.), and irradiated with 40 W ultrasonic waves for 360 seconds at a flow rate of 40%, and then the particle size distribution was measured using a laser diffraction particle size distribution analyzer "MT3000II" manufactured by Nikkiso Co., Ltd. The particle size D ５０ was measured.

[0067] [Conductivity] The solid electrolyte was measured in a glove box purged with sufficiently dried Ar gas (dew point -60°C or less) to obtain a conductivity of about 6 t / cm ２ The pellets were subjected to uniaxial pressure molding under a load of 1000 kJ / cm2 and approximately 0.5 to 8 mm thick to prepare samples for measuring lithium ion conductivity. The lithium ion conductivity of the samples was measured using a Solartron 1255B impedance measuring device manufactured by Toyo Corporation. The measurements were performed by the AC impedance method at a temperature of 25°C and a frequency of 0.1 Hz to 1 MHz.

[0068] [Reaction initiation temperature with manganese dioxide] Spinel-type lithium manganese dioxide (LiMn) is known as a positive electrode active material for lithium batteries. ２ O ４ ) and manganese dioxide (MnO) as a model compound of rock salt layered type positive electrode active material. ２ The reactivity of manganese dioxide with the solid electrolytes obtained in the examples and comparative examples was evaluated using a manganese dioxide sample prepared by the following method. The manganese dioxide simulated a charged positive electrode in which lithium ions had been desorbed from a spinel-type lithium manganate or a rock salt layered positive electrode active material. Manganese dioxide (manufactured by Kojundo Chemical Co., Ltd.) and the solid electrolyte were mixed in a mass ratio of 70:30 and sieved using a 53 μm sieve. 2 mg of the mixture that passed through the sieve was filled into an aluminum pan and subjected to differential thermal analysis. The temperature range of the differential thermal analysis measurement was 25°C to 500°C. The differential thermal analysis was performed using a STA 2500 Regulus (manufactured by NETZSH Co., Ltd.). The heating rate was 10°C / min. The measurement atmosphere was nitrogen flow. The peak-top temperature of the exothermic peak observed by differential thermal analysis was measured and used as the reaction initiation temperature with manganese dioxide. The higher the reaction initiation temperature, the less the solid electrolyte reacts with manganese dioxide, that is, the less the solid electrolyte reacts with the active material.

[0069]

[0070] As is clear from the results shown in Table 1, the solid electrolytes obtained in each Example had a higher reaction initiation temperature with manganese dioxide than the solid electrolytes obtained in the Comparative Examples. This shows that even if a solid battery containing the solid electrolyte obtained in each Example is unintentionally exposed to a high-temperature environment in a charged state, the solid electrolyte is less likely to react with the active material, and the safety of the solid battery is high.

[0071] As described above in detail, the present invention provides a solid electrolyte that enables a solid-state battery to be used safely even when the environment changes from a normal environment to a severe environment.

Claims

1. In the X-ray diffraction pattern measured by an X-ray diffractometer using CuKα radiation, the integrated intensity of the diffraction peak observed at 2θ = 15.4° ± 1° is defined as I. Ａ The integrated intensity of the diffraction peak observed at 2θ = 17.8° ± 1° is defined as I Ｂ The integrated intensity of the diffraction peak observed at 2θ = 25.3° ± 1° is defined as I Ｃ The integrated intensity of the diffraction peak observed at 2θ = 29.7° ± 1° is defined as I Ｄ When this is done, I Ａ / I Ｂ The value of is 1.0 or more, and I Ｃ / I Ｄ A solid electrolyte having a value of less than 0.

90.

2. In the K-edge spectrum of sulfur element obtained by the total electron yield method of X-ray absorption fine structure, the maximum peak intensity of 2471 eV ± 1 eV is I Ｘ The maximum peak intensity of 2475 eV ± 2 eV is I Ｙ When I Ｘ / I Ｙ A solid electrolyte having a value of 0.89 or less.

3. The solid electrolyte according to claim 1 or 2, which contains lithium (Li), phosphorus (P), sulfur (S) and halogen (X), and the molar ratio of halogen (X) to phosphorus (P) is less than 1.

2.

4. The solid electrolyte according to claim 3, wherein the molar ratio of sulfur (S) to phosphorus (P) is greater than 4.

8.

5. Volume cumulative particle size D at 50% cumulative volume by laser diffraction scattering particle size distribution measurement method ５０ 2. The solid electrolyte according to claim 1, wherein the X-ray diffraction pattern has a half-width of a diffraction peak observed at 2θ=29.7°±1° of 0.16° or less.

6. The particle diameter D relative to the crystallite size (Å) determined based on the X-ray diffraction pattern ５０ (μm) 10 ４ The solid electrolyte according to claim 5, wherein the ratio is 1 or more and 15 or less.

7. The solid electrolyte according to claim 1 or 2, which contains a crystalline phase having an argyrodite-type crystal structure.

8. An electrode mixture comprising the solid electrolyte according to any one of claims 1 to 7 and an active material.

9. A solid electrolyte layer containing the solid electrolyte according to any one of claims 1 to 7.

10. A solid-state battery having a positive electrode layer, a negative electrode layer, and a solid electrolyte layer located between the positive electrode layer and the negative electrode layer, the solid-state battery containing the solid electrolyte according to claim 1 or 2.

11. A method for evaluating a solid electrolyte, in which a transition metal oxide and a solid electrolyte are mixed, differential thermal analysis is performed, and the oxidation resistance of the solid electrolyte is evaluated based on the temperature at which an exothermic peak occurs.

12. The transition metal oxide is MnO ２ , Mn ３ O ４ , Mn ２ O ３ , MnO, Mn ２ O ７ , NiO ２ The evaluation method according to claim 11, wherein the metal oxide is NiO or NiO.

13. The evaluation method according to claim 11 or 12, wherein mass spectrometry is carried out simultaneously with the differential thermal analysis measurement to evaluate the type and amount of gas generated from the solid electrolyte.

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